This is a Dyson style post-motor pleated filter assembly for the Two Speed V6 battery operated stickvac vacuum cleaner. It's in a bright purple and fits at the back of the motor on the pistol grip, complete with the HIGH/LOW thumb press switch.
They click off and on with a short 10 degree twist. It's very hard to see how contaminated it is, the performance is best judged by comparing the back pressure from the new filter with the back pressure of the old filter. Offer the filters to the air blowing out of the back of the motor to feel the difference!
If the filter is badly blocked, it will cause the stickvac to stop. It's partnered with pre-filter